In collaboration with Kevin Hall, I have been interested in modeling how the body partitions diet macronutrients into lean and fat body tissue. Using empirical cross-sectional data of the relationship between fat and lean mass, we have developed an expression for how substrate utilization adapts to changes of diet, energy expenditure, and body fat so that energy imbalances produce the observed changes of body composition. The theoretical prediction matches experimental data without the use of free parameters. [unreadable] [unreadable] In collaboration with Anne Sumner and Vipul Periwal, I have been developing a time dependent mathematical model of the suppression of lipolysis by insulin. The model can be used to develop a quantitative measure for insulin's effect on serum free fatty acids akin to insulin sensitivity for glucose. We have tested 7 possible models and used a Bayes factor analysis to choose the model that best balances fit to data with minimal model complexity.[unreadable] [unreadable] With postdoctoral fellow Wei Huang, I have been exploring the early stages of differentiation in embryonic stem cells. We have found evidence that the changes in expression of certain genes involved in maintaining pluripotency may be reversible by the administration of different factors. Our goal is to extract the gene regulatory circuitry involved in these genetic and cellular processes. A confounding problem is that the cells both divide as well as differentiate. Our method is to fit a growth-drift-diffusion model to the data to distinguish between the two effects.[unreadable] [unreadable] In collaboration with Stoney Simons, post-baccalaurete fellow Karen Ong and I are developing a biophysical model for steroid-regulated gene expression in the presence of various factors. Experiments have found that factors can alter both the EC-50 and Vmax of the final product. This puts a strong constraint on the possible mechanisms. We have developed a model involving a cascade of reactions that can predict the actions of UBC-9, which is an enzyme important for sumoylation.